1. Field of the Invention
The present invention concerns a radio-frequency transmission arrangement for a magnetic resonance system for generation of a B1 field.
2. Description of the Prior Art
Using magnetic resonance tomography it is possible to generate slice images of the human body in arbitrary planes by selected excitation of the magnetization of nuclear spins in examination subject located in an optimally homogeneous, static basic magnetic field (also designated as a B0 field). To excite the magnetization resulting from the nuclear spins, radio-frequency pulses are radiated into the examination subject with radio-frequency antenna. The electromagnetic flux density of these radio-frequency pulses is typically designated as a B1 field. Usually the specification of the radio-frequency pulses necessary for magnetic resonance excitation is provided digitally by control electronics as a sequence of complex numbers. These are supplied from a modulator and an RF power amplifier to a transmission antenna.
Conventional magnetic resonance systems have only one whole-body antenna that should generate an optimally homogeneous B1 field. Since the tissue of the person examined in the MR system is conductive, the induced currents alter the inherently homogeneous B1 field distribution. Temporal and spatial fluctuations in the field strength of the excited B1 field lead to changes in the acquired MR signal, which leads to unwanted changes of the image intensity in the shown MR image. For example, an inhomogeneous spatial distribution of the amplitudes of the B1 field leads to an unwanted dependency of the image contrast on the spatial position of the signal sources (excited nuclei). For this reason transmission systems with a number of channels are desirable, which respectively exhibit different field distributions, such that an essentially homogeneous B1 field can be generated by superimposition of the various fields. DE 10 2004 053 777 A1 describes such a transmission arrangement with a number of channels.
Furthermore, new acquisition techniques have been developed with which different B1 field profiles that are generated by a number of antennas are used simultaneously. For this reason it is desirable to use a radio-frequency transmission arrangement that can generate various field distributions. In conventional magnetic resonance systems only one whole-body antenna is typically installed that can generate only one (preferably homogeneous) field distribution. So that users of systems with installed whole-body antennas can make use of applications with a number of transmission channels, radio-frequency transmission arrangements would thus have to be retrofitted with a number of channels. This is very costly and time-intensive.